About Capacitors

| February/March 2003

  • SmokesStak

  • SmokesStak

A recent topic on the SmokStak bulletin board at brings up an item that is responsible for spark plug ignition failure -the capacitor.

I would like to offer my help to those folks who may not understand capacitors, also commonly called condensers.

First off, what is a capacitor? A capacitor consists of two metal plates separated by an insulating material, such as paper. Engine capacitors are usually two long strips of aluminum foil about an inch wide sandwiched between layers of waxed paper, then rolled up and placed in a metal case. One plate is connected to the case and the other to a lead out wire. Fine, but what does it do?

When a capacitor is connected to a voltage source, such as a battery, it acts momentarily as a closed circuit as current flows to charge the plates; negative electrons on one plate and positive on the other. When the voltage across the plates builds up to the battery voltage, current flow stops and the capacitor acts as an open circuit. The plates remain in a charged condition even when disconnected from the battery. A capacitor, therefore, stores electric power just as a battery does.

But, if a circuit path is connected to the plates the capacitor will discharge its stored energy through that path. And, if the voltage polarity is reversed it will recharge to the opposite polarity.

So, how do you test a capacitor to see if it's good? If you have an ohmmeter, connect the test leads together and zero the meter. If the meter won't zero replace its battery. Select the highest R times range you have; R times 10,000 works well. Connect the test leads to the capacitor while watching the meter. Movement on the scale means the meter is taking the charge, and prompt movement back to full left on the scale means there are no leaks in the capacitor. A leak is a small amount of continuous current flowing through a high resistance bridge across the plates. Don't touch the conductors, because you will read leakage through your hands. Repeat the test while reversing the test leads, and it's a good idea to test several known good capacitors to establish a reference. - Ralph


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